1. Field of the Invention
The present invention relates to an electricity storage device and a device using the same, and in particular, to a rechargeable electricity storage device which is preferably used in applications with pulsed load current and a portable electric-powered tool equipped with the same.
2. Description of the Prior Art
Portable and rechargeable electric-powered tools, such as cordless drills, electric saws or electric nail-puncher, need battery packs for supplying electricity. A battery pack forms an electricity storage device As shown in FIG. 1, a conventional battery pack, i.e. an electricity storage device 1, includes a plurality of batteries 2 connected in series and an electricity output end 3. A working voltage is achieved by cascading the plurality of batteries. Conventionally, a nickel-cadmium battery is used which can produce a large quantity of instantaneous current, that can be as high as twenty times its capacity, and is used for supplying pulse loading current. However, the energy density of the nickel-cadmium battery is not high enough, therefore it is heavy and has less available working time. A lithium battery such as a lithium ion or lithium polymer battery, though possessing advantages of light-weight and high energy density, is not suitable for large current discharging, and thus cannot be used in applications of pulsed load current. Therefore, conventional battery packs and portable electric-powered tools hold drawbacks of heavy weight and shortage in available working time.
Conventional battery packs and portable electric-powered tools are also strictly restricted by the characteristics of a secondary battery. Besides the disadvantages that the battery can not release electricity in high rates rand has limited power output, more problems arise especially in an application with pulsed load current. For examples, high electricity outputs will reduce the capacity and cycle life of the secondary battery, and usually the battery is recharged in an incomplete discharge state, which results in a severe memory effect and drastically reduces the battery capacity and life.
Connecting a capacitor to the battery is a solution to meet the requirement of a large and instantaneous current output. The battery may store a large quantity of electricity, but can not deliver the electricity very fast and is limited in output power. On the other hand, the capacitor can provide a large quantity of current, but can not last for a long time and its energy density is much lower than the battery. Among all types of capacitors, a super capacitor is suitable in an application of pulse loading current since it has a higher power, voltage and energy density than an ordinary capacitor. In a battery pack which includes a battery and a super capacitor, the battery mainly charges the super capacitor and the super capacitor mainly supplies a large current in an application of pulse loading current. Whereby, a wide variety of batteries can be used and the life and efficacy of the battery pack can be well ensured.
Conventional connections of batteries 2 and a super capacitor 4 are shown in FIG. 2 and FIG. 3. In these prior techniques, before connecting the battery 2 and the super capacitor 4, either a converter 5 as shown in FIG. 2, or an diode 6, current-limiting switch 7 and a current-limiting resistor 8 as shown in FIG. 3 are connected to avoid explosions of the super-capacitor. The conventional connections hold the disadvantages of high cost, complex and difficult in assembly.
The present invention aims to overcome problems of the prior art and discloses an electricity storage device and a portable electric-powered tool which perform efficiently, have an extended life and are not restricted by the battery types.
It is an object of the present invention to provide an electricity storage device which can meet the requirement of instantaneous high power output and extend available working time. The battery capacity can be fully utilized and the battery life can be extended. In addition, the cost can be reduced and the assembly is simple.
It is a still object of the present invention to increase varieties of batteries in an application of high power output. Not only the nickel-cadmium battery, but also high energy density batteries such as nickel-metal hydride batteries, lithium-ion batteries and lithium polymer batteries can be used without any modifications. Even for an ordinary battery, it does not need to sacrifice its energy density in meeting any requirements of increasing power density.
It is a further object of the present invention to provide a more convenient and lighter battery pack which has higher capacity and can be used for more times and avoids memory effect.
It is a further object of the present invention to provide a portable electric-powered tool which utilizes the above electricity storage device and has advantages thereof
To achieve the above objects and avoid the disadvantages of the prior art, the present invention discloses an electricity storage device which comprises at least one battery; at least a super capacitor which has a less internal impedance when fully charged than that of the battery and connects the battery in parallel; and an output end for supplying the electricity; whereby, the super capacitor is the major electric power supplier of pulse current output and the battery is used for providing electricity to the super capacitor and is the minor electric power supplier of pulse current output. The connection of the battery and the super capacitor does not need any convectors or current-limiting resistors.
In embodiments, the battery types include lithium battery, nickel-cadmium battery, nickel-zinc battery and nickel-metal hydride battery. The lithium battery is preferable since it is light, has a high capacity, can be repeatedly used for an extended period of time and does not have memory effect. In an application where a plurality of batteries are required, the batteries are connected to each other in series.
In another embodiments, the electricity storage device of the present invention further comprises a battery protection and control device, and a bi-directional switch which connects the battery in series. The battery protection and control device is used for controlling the switch of the bi-directional switch. Whereby the battery is protected from being overcharged, being over discharged and being operated in the condition of large discharging current.
The electricity storage device may further comprise a zener diode which connects the super capacitor in series. Therefore, the super capacitor is prevented from being operated at over voltage and the energy storage device is prevented from being operated at over current.
The present invention also discloses a portable electric-powered tool, which cannot be separated from the electricity storage device in practical applications of pulse loading current output, comprising a tool body and a battery pack for supplying electricity to the tool body; the battery pack comprises at least one battery and at least a super capacitor which connects the battery in parallel; wherein the super capacitor has lower internal impedance when fully charged than that of the battery; whereby, the super capacitor is the major electric power supply for pulse current output and the battery is used for providing electricity to the super capacitor and is the secondary power supply for pulse current output. The connection of the battery and the super capacitor does not need any converters or current-limiting resistors.